A step transformer has a rotor that is moved angularly in steps to switch the transformer output and/or input to different taps on the transformer. A terminal must be moved from one tap to the next one with the highest possible starting speed to avoid drawing an arc or to break any arc drawn as quickly as possible.
It is therefore common to provide a spring-loaded force-storing arrangement. Movement of an input element in a direction intended to switch the rotor of the transformer is first merely transferred to this force-storing device to compress and/or extend the spring or springs thereof. Once a critical point is reached, this stored-up spring force is released to snap the rotor angularly to the next position. This therefore allows a relatively slowly moving motor to drive the rotor of the step transformer with the desired snap action.
This can be done as described in German patent 857,519 of B. Jansen by actuating the rotor through a changing-length lever arm. It is therefore possible to achieve the desired snap action, but the mechanism for doing so is fairly complex and expensive because the various guides and the like must be made to fine tolerances. A similar system is described in German patent 1,184,580 of Al Bleibtreu which uses a spring arrangement and a complex system of levers and roller guides.
German patent 2,719,396 uses an additional spring in the force-storing arrangement. This extra spring only is effective in the last portion of the tensioning operation and thus is effective only at the start of the release movement so as to achieve a high starting speed. Once again, the use of this extra spring complicates the mechanism of the device and requires that a fairly strong main drive motor be used.
Accordingly German patent 2,250,260 of E. Baumgartner describes a system with two coaxial levers having ends interconnected by springs. A snap-action cam is effective between these levers and the rotor shaft of the step transformer. In this arrangement the springs remain substantially parallel to each other so that force is not accurately transmitted to the levers.
Similarly in German patent document 2,337,658 of F. Pelz parallel springs are employed which are stretched from different points. On tensioning a gear wheel rolls on an inner ring gear of the housing so as to tension a spring that is only released at the end of its travel. In these arrangements the springs are stressed within the limits of their elasticity, that is in the straight parts of their response curves. Unfortunately considerable spring force is lost to the various elements that must be actuated to eventually move the actual switch rotor, as the springs are invariably stressed diametrally while a rotary force is needed to operate the step transformer.